In an ordinary serial printer, a print head is located opposite to a circular platen and is disposed for movement back and forth along a platen shaft. A printing paper is supplied to a paper feed mechanism disposed around the platen in a manner such that its leading edge is disposed against the back surface of the platen which is opposite from the print head so that it is fed together with the platen as the latter rotates to a position which is located opposite to the print head. Accordingly, a paper sensor which is used to detect the leading and the trailing edge of the paper cannot be disposed in a region forwardly of the front platen surface across which the print head moves back and forth. Certain type of ink jet printers are disclosed in Japanese Laid-Open Utility Model Applications No. 60,505/1979 and No. 17,574/1980, for example. In such instance, the usual practice is to dispose the paper sensor rearwardly of the platen since the areas around the platen may be contaminated by ink sprays from the ink jet.
When the paper sensor is located rearward of the platen, there exists an increased spacing between the print position or the location of the print head and the location where the presence of a paper is detected by the paper sensor, and hence a signal from the paper sensor cannot be directly used in providing an accurate control over paper feed or recording operation. To illustrate, it is possible that the paper sensor has detected the presence of a paper, but there is no paper present at the location where a recording operation by the print head should take place. Conversely, it is also possible that the paper may be present at the location for recording by the print head even though the paper sensor does not detect the presence of a paper. A similar problem occurs when automatically loading a printing paper around the platen.
In an ink jet printer of charge controlled type, a jet of ink is continuously projected from a nozzle, and substantially all of the ink which has not been caused to impinge upon a recording paper is recovered by a gutter to be returned to an ink reservoir. Such portion of the ink which is not directed toward the recording paper flies through the air, after being projected through the nozzle and before impinging upon the gutter, and experiences an evaporation of a solvent to result in an increased ink viscosity. Also, oxidation may cause a modification or degradation in the ink quality.
To reduce a change in the ink viscosity, or a modification and/or degradation in the ink quality, it is proposed in Japanese Patent Publication No. 36,863/1982 that when data to be printed ceases to be supplied for a given time interval, the drive of a pump is interrupted and a solenoid valve is switched to establish a direct communication between the nozzle and an ink reservoir, thus ceasing the projection of an ink jet and returning the ink from the head to the reservoir.
However, in a printer, there is a frequent occurrence that a printing operation must be initiated immediately after a given time interval in which the data to be printed ceases to be supplied. The projection of an ink jet is preferably continued even though the data to be printed ceases to be supplied over a given time interval, if a time-over occurs during the time that a line feed operation is repeated or as a result of retarded page-out rate. If the projection of an ink jet is once interrupted, the re-initiation of the projection of an ink jet is followed by a standby time during which an ink pressure, an ink temperature and other parameters are monitored for a given time interval until they assume given values, whereupon the retrieval of phase or a control of deflection is made, all of which contribute to retarding the initiation of a recording operation. Viewed differently, a host unit such as a computer, a word processor or a scanner which supplies data to be printed to the printer produces an initializing command which indicates the re-initiation of a printing operation, and the printer initiates the projection of an ink jet in response thereto. After a standby time of a given length, the printer performs a phase retrieval and a deflection control before it transmits a ready signal to the host unit, clearing a busy signal which has been transmitted to the host unit, thus initiating a control over the printing operation. Thus, tasks increase on the part of the host unit. Also, the printer must perform an initializing control, similar to that during an initial warm-up period when the power is turned on, thus also increasing the tasks on the part of the printer. If it is attempted to allow a rapid rise time control, especially for the interruption of an ink jet which occurs as a result of the cessation of data to be printed, this increases the program on the part of the printer, increasing the control tasks.
It will be seen that the host unit delivers an initializing signal to the printer only after the transfer of data to be printed is possible, and hence as viewed from the host side, it must wait for a standby time of a substantial length since the condition on the part of the host unit permits a printing operation. Where an operator has a knowledge about the condition of the host unit or is operating it, the operator can predict the time when the transfer of data to be printed from the host unit is possible. Accordingly, it is preferred that the ink jet printer be previously activated by a manual command to establish a condition in which an ink jet is available. In such instance, a signal which indicates the interruption of an ink jet, a signal which indicates that the projection of an ink jet is being interrupted, and also a signal that the re-initiation of an ink jet has enabled a printing operation be preferably produced.
On the other hand, the ink viscosity tends to increase during the time the ink jet is being projected, thus causing a modification or degradation in the ink quality. Accordingly, it is preferable that an ink in a circulating system be discarded in small quantities in order to expedite the consumption of the ink.
It will be understood that a rise time which is required during the standby mode which immediately follows the momentary interruption of the ink jet can be reduced as compared with the rise time required immediately after the power turn-on of the printer, and it is desired that such rise time be minimized. Also when the printer is conveyed, air bubbles will be mixed with the ink, requiring an ink aging projection which must be continued over a relatively long period. It will then be noted that the interruption of an ink jet as a result of the cessation of the data to be printed causes such aging projection to be interrupted, and hence a wearisome operation may be required to provide an initializing command in response to each interruption of the ink jet. Alternatively, a separate approach must be provided.